(1) Field of the Invention
The present invention relates to a color image forming apparatus and in particular relates to a color image forming apparatus, such as a copier, printer, facsimile machine or the like, which uses electrophotography as a process of image forming, wherein an intermediate transfer medium of an endless belt is used for transferring developer images formed on image bearing members.
(2) Description of the Prior Art
Recently, in the field of image forming apparatus, there is a common trend toward color configurations, and with the development of color image forming apparatus, an increased number of color image forming apparatus have become used.
As one of the color image forming systems, a configuration using an endless intermediate transfer belt has been known in which developer images (color images of information formed with multiple colors of developers) formed on photoreceptor drums by image forming means are transferred to the intermediate transfer belt (primary transfer) and the lamination of the developer images transferred on the intermediate transfer belt is further transferred to the paper (secondary transfer).
Usually, in a color image forming apparatus using the intermediate transfer scheme including primary and secondary transfer stages, the applied electric fields generally are specified so as to satisfy the following relationship: [(primary transfer electric field)×0.5]<(secondary transfer electric field)<[(primary transfer electric field) ×1.7].
In order to preferably transfer the developed image primarily transferred on the intermediate transfer belt to the paper at the time of secondary transfer, it is necessary to negate the effect of the electric field at secondary transfer until the intermediate transfer belt reaches the first point at which the primary transfer is performed next (the primary transfer station, downward of and closest to the secondary transfer station with respect to the moving direction of the intermediate transfer belt).
For this purpose, there have been known color image forming apparatus using an intermediate transfer belt, in which an intermediate transfer belt erasing device (applying an erasing electric field or grounding the belt) is disposed between the secondary transfer and the primary transfer with respect to the moving direction (direction of travel) of the intermediate transfer belt, by taking the belt characteristics (attenuation characteristics as to the applied voltage) into account (see Japanese Patent Application Laid-open Hei 11 No. 167294).
The erasing device stated above, however, not only promotes enlargement of the apparatus but also unable to achieve perfect removal of the static charge from the intermediate transfer belt. Specifically, the static charge on the top surface of the intermediate transfer belt can be erased by the aforementioned erasing device, but the charge dwelling in the interior of the intermediate transfer belt cannot be erased, and the static charge moves up to the top surface as time passes. Resultantly, the top surface, being at a time erased, will bear static charge once again.
If image forming is implemented continuously with this situation left as is, static electric charge will build up on the intermediate transfer belt, inducing a high voltage and high electric field. As a result, there occurs the problem of transfer failures at the times of primary and secondary transfer.